Polyimides have been widely used in the electronics industry as a film coating material on metal substrates. Polyimides are well known for their excellent heat-resisting characteristics. However, there exist several areas that need to be improved regarding the application of polyimides in the electronics industry. Typical of these problem areas include: the degree of adhesion between polyimides and the metal substrate (for example a copper foil), the dimensional stability of the electrolytic copper laminate, and the flatness of the copper laminate surface after being clad with polyimides. Surface fatness determines the closeness at which the circuit line can be packed together. A highly fiat surface (i.e., a surface with large curvature radius) will be required in order to provide a fine line circuit.
A common approach to improve the degree of adhesion between polyimides and the copper foil is to use adhesives. Proper selection of adhesives can also help maintaining the flatness of the copper foil surface. However, most adhesives do not have the same extent of heat resistance as polyimides. The use of adhesives often reduces the temperature range of the final product and, therefore, limits the types of applications that polyimides may be used. The additional step of having to apply adhesives before applying the polyimide coating layer also complicates the manufacturing procedure in making copper clad laminates. Therefore, it is desirable to develop polyimide compositions that provide good adhesion characteristics between the coating layer and the copper foil without the need to use adhesives, and, at the same time, ensure that the final product has acceptable surface flatness. Furthermore, the use of an adhesive layer increases the thickness of the copper clad laminates.
As opposed to the three-layer (polyimide-adhesive-copper) copper clad laminates described above, two-layered laminates can be prepared which involves applying a polyimide precursor, typically polyamic acid, on the copper foil, then heating the coated copper foil to remove solvent and effectuate curing of the polyamic acid into polyimide. The two-layer process allows better manufacturing efficiency to be obtained. However, the curing reaction from polyamic acid into polyimide requires elevated temperature (&gt;300.degree. C.) to achieve complete reaction. This can cause production difficulties as well as wasting energy and requiring excessive insulation. Furthermore, at elevated temperatures, copper foil can be oxidized and darkened. These concerns discourage the curing of the polyamic acid-coated copper foil to be conducted in conventional open-air furnaces, and cause difficulties in designing a continuous process for production operations.
U.S. Pat. No. 5,077,084 discloses a process for producing a flexible printed base by directly coating a copper foil with a polyimide precursor, following by heating, drying, and curing in an inert gas furnace at temperatures of 200.degree. and 450.degree. C. The main disadvantage of oven-curing of polyimide, i.e., the oxidation of the copper foil, has been discussed above. In order to obtain desirable foldable of the flexible printed base, the coated copper foil must maintained at a controlled tension of between 0.02.about.0.2 Kg/cm.sup.2, and the oxygen concentration must be maintained below 0.2%.
J. G. Stephanie and P. G. Rickerl, in an article entitled "Infrared Curing Polyimides", ANTEC pp. 1696-1699 (1991), disclosed a process of curing polyimide in an infrared oven with a 4-zone heated chamber, having 56 quartz-tungsten lamps above and below a nichrome belt. To ensure temperature uniformity inside the oven, a porous quartz muffle is provided to allow a very diffuse nitrogen gas flow. The infrared oven used in Stephanie, et al. provides essentially the same function as does the conventional oven used in the '084 patent, except that the electric heating element of the '084 patent is replaced with a filament of tungsten alloy; both of which are used as the energy source to maintain the oven at the controlled temperature.
The traditional oven-curing of polyimide, whether it be the conventional oven or the infrared oven, does not submit a very efficient use of energy. Furthermore, the heating and cooling steps involved in the oven-curing process are often time-consuming, and often impede the implementation of continuous operation. Therefore, it is desirable to develop improved techniques that can be used for curing polyimide in making copper clad laminates.
Several methods have also been disclosed in the prior art to improve physical characteristics of the two-layered polyimide-copper clad laminates. In an article entitled "New Semi-Interpenetrating Polymeric Networks from Linear Polyimides and Thermosetting Bismaleimides: 2. Mechanical and Thermal Properties of the Blends," authored by T. Pascal, et at, POLYMER, vol. 31, pp 78-83 (1990), bismaleimide was studied as an additive to be blended with polyimides to improve the mechanical properties, such as tensile strength, thereof. However, to use the polyimide-bismaleimide blend as a coating composition, other factors must also be considered, most notably the surface flatness.
In Republic of China, Utility Pat. No. 158,619 (the '619 patent), it was disclosed a bismaleimide and polyimide mixture composition for coating copper foil. The degree of adhesion, measured in terms of peel strength, was increased from 4.5 lb/in when using polyimide alone, to 8 lb/in using the mixture composition. However, the coating composition disclosed in the '619 patent does not address the issue of surface flatness. Surface curling of the copper foil, which is an indication of imperfect surface flatness, was a major problem with the mixture composition disclosed therein.
Japanese Patent App. JP 91-123,093 discloses a method for the manufacture of copper-clad laminate with multi-layer polyimide films for flexible printed circuits. An electrolytic copper foil was coated with more than two layers of polyimides having different thermal expansion coefficients. The inventors asserted that the final product showed no cuff; however, no mentioning was made regarding the degree of adhesion between the polyimide coatings and the copper foil.